1. Field of the Invention
The present invention relates to brightness control of a monitor, and more particularly to an automatic brightness control apparatus for a monitor which can provide a high picture quality by automatically adjusting the brightness of a back raster accordance with the brightness of a picture on a screen.
2. Description of the Prior Art
A conventional brightness control apparatus for a monitor, i.e., a cathode-ray tube (CRT) is illustrated in FIG. 1. Referring to FIG. 1, the conventional apparatus is provided with a flyback transformer T1 having one secondary winding for generating a CRT anode voltage and the other secondary winding for generating a CRT grid voltage, a diode D1 for passing negative-going portions of pulses induced in the secondary winding of the flyback transformer T1 for generating the CRT grid voltage, and a capacitor C1 for smoothing the voltage provided through the diode D1.
The conventional apparatus is also provided with a first variable resistor VR1 for adjusting the smoothed voltage to provide the adjusted voltage to a grid terminal G of the CRT, a second variable resistor VR2 for adjusting an input fixed voltage Vo to provide the adjusted input voltage to the grid terminal G of the CRT, and a blanking circuit section 1 for providing blanking pulses to the grid terminal G of the CRT.
In FIG. 1, the numerals C2 and C3 denote capacitors, D2 and B3 denote diodes, and R1 denotes a resistor.
The operation of the conventional brightness control apparatus as constructed above will now be explained.
If the B.sup.+ supply voltage is applied to a primary winding of the flyback transformer T1 by switching operation of a transistor Q1, a high voltage is induced in one secondary winding of the flyback transformer T1 for generation the CRT anode voltage, and the induced high voltage is applied to the anode of the CRT.
At the same time, a voltage induced in the other secondary winding of the flyback transformer T1 for generating the CRT grid voltage is supplied to the diode D1, so that the diode D1 passes and provides to the capacitor C1 only negative-going portions of the voltage.
The capacitor C1 smooths the negative-going voltage portions provided from the diode D1 and provides the smoothed DC voltage to the first variable resistor VR1. The negative DC voltage from the first variable resistor VR1 is then supplied to the grid terminal G of the CRT.
The first variable resistor VR1, which is installed outside the monitor, is adjusted by a user's manipulation thereof to vary the voltage level being supplied to the grid terminal G of the CRT.
The second variable resistor VR2, which is installed inside the monitor, is for varying the input fixed voltage Vo to supply the varied input voltage to the grid terminal G of the CRT. The blanking circuit section 1 generates and provides blanking pulses to the grid terminal G of the Crt through the capacitor C2.
Accordingly, the voltages adjusted by the respective variable resistors VR1 and VR2 as well as the blanking pulses are supplied to the grid terminal G of the CRT. Specifically, the adjusted voltages through the variable resistors VR1 and VR2 are for controlling the brightness of the picture produced on the screen, while the blanking pulses are for blanking the retrace lines appearing on the screen during retrace periods.
According to the conventional brightness control apparatus as described above, since the brightness of the back raster on the monitor is fixed constant by the adjustment of the variable resistors, the back raster brightens, causing the picture to become brighter, when a high-leveled beam current flows to the anode A of the CRT to represent a bright image, while the back raster darkens, causing the picture to become darker, when a low-leveled beam current flows.
Further, the conventional apparatus has the drawback that the picture represented at the center portion of the screen becomes brighter than that at the circumferential portion thereof due to the difference in deflection angle between the center and the circumferential portions.